A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface, traditionally as a series of closely spaced adjacent two-dimensional (2D) lines. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. (Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profiles) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
The data in a 3D survey are amenable to viewing in a number of different ways. First, horizontal “constant time slices” may be taken extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2D plane of seismic data. By animating a series of 2D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2D seismic line from within the 3D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
However, for all of the advances that have been made in recent years in the technology of seismic processing, the resulting image of the subsurface is often compromised by the ability to get geophones into position to receive the returning subsurface signals. In more particular, in rugged terrain it may be difficult to maneuver a conventional seismic line into position so that receivers can be placed where they need to be in accordance with the survey plan. Additionally, as the length of the seismic line increases (because of increased geophone spacing, increased number of channels, and/or longer offsets) the weight of the cable that connects each geophone (or geophone array) to a central recording unit becomes increasingly burdensome and more difficult to maneuver. The net result of the above is poorer coverage (because of missing receivers) in rugged terrain and increased expense.
Of course, this aspect of seismic data collection has long been understood. It was recognized early on that if it were possible to eliminate the interconnecting cable (other things being equal) the deployment and retrieval costs would be substantially less and, in some cases, fewer field personnel would need to be employed. In view of these and other advantages, it is not surprising that there have been numerous attempts to create a wireless system. However, current solutions to this problem have not proven to be entirely satisfactory.
The state of the art in wireless (or cableless) systems involves the use of two-way radio communications between geophones and a central recording facility. In simplest terms, radio-based wireless systems equip each geophone (or seismograph) with its own power supply and a radio transmitter/receiver (i.e., “transceiver”). Seismic signals that are recorded by each geophone are transmitted to a central receiver (which is a transceiver in most instances) for recording onto magnetic tape or disk (a base station, hereinafter). Additionally, each geophone might be equipped with some amount of RAM (or flash RAM, disk, etc.) in which to store the in-coming seismic data until such time as it can be transmitted to the central recording facility for long-term storage. It should be noted that in most circumstances there is insufficient bandwidth to simultaneously stream the seismic data from all of the (potentially many thousands of) geophones at once. Instead, in most cases each receiver has enough internal storage to record at least a few shots before needing to transmit to base station.
In many systems, the base station also transmits commands to each seismograph. These commands might include simple directives such as “start recording”, “start/stop recording”, “sleep” (i.e., switch to power saving mode), “wake”, etc. Additionally, it is not uncommon to configure the seismographs to respond to commands such as “send status”, “begin upload”, etc., where a reply from the seismograph is expected. Among the sorts of replies that might be transmitted include verification of electrical integrity, available storage levels, “help me I'm broken”, etc.
Communication between the recording station and remote seismographs (e.g., “field units”, “channels”) is obviously more difficult with wireless systems than it is with conventional cabled systems. Typically, transmission of control signals, timing signals, electrical performance data, quality control information, seismic data, etc. must either be foregone or restricted and, in some cases, such information can only be transmitted via a separate radio system (includes wireless LANs). Thus, alternative solutions have been, and continue to be, sought.
Among the alternative approaches that have been tried are low frequency transmitters/receivers and WiFi. However, there are problems with these sorts of approaches. As an example, since a typical seismic survey may utilize several thousand geophones at any one time transferring the data from each geophone to the base station can prove to be problematic. In the case of a WiFi approach, communications tend to break down when thousands of receivers try to transmit seismic data simultaneously to a base station. In brief, neither of these methods has been particularly successful.
Another disadvantage of land cableless seismic systems is that the remote seismographs are easily stolen. The loss of the instrument is serious enough, but the cost of the data stored within the memory is typically hundreds or thousands of times greater than the cost of the instrument.
Of course, a land cableless seismic system has less weight, lower capital investment requirements, and generally lower operating costs. Finally, cableless systems are much preferred over cabled systems as they have a reduced impact on the environment. Obviously, laying out (which may require some foliage reduction) and then collecting miles of heavy seismic cables has an increased potential to damage vegetation and other aspects of the wilderness environment as compared with cableless system, thus making them increasingly attractive.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a cableless seismic system that does not suffer from the disadvantages of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a method of seismic data acquisition that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.